US 20100055580 A1
A method for manufacturing a semiconductor device using a photomask and optical lithography is disclosed, wherein circular patterns on the semiconductor wafer are formed by using circular patterns on the photomask, which is manufactured using a charged particle beam writer. In one embodiment, circular patterns of varying sizes have been formed on the photomask using a single character projection (CP) character, by varying the charged particle beam dosage. A method for fracturing circular patterns is also disclosed, either using circular CP characters or using VSB shots wherein the union of the plurality of VSB shots is different than the set of desired patterns.
1. A method for manufacturing a semiconductor device on a substrate comprising:
providing a photomask, wherein the photomask comprises circular patterns, and wherein the photomask has been manufactured using a charged particle beam system; and
using optical lithography to form a plurality of circular patterns on the substrate using the circular patterns in the photomask.
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6. A method for fracturing or mask data preparation for charged particle beam lithography comprising:
inputting a set of patterns to be formed as circles on a surface;
determining a set of shots that can form the set of circular patterns on the surface, wherein the dosages of the shots in the set of shots may vary with respect to each other; and
outputting the set of shots, including the dosages.
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14. A system for fracturing or mask data preparation for use with charged particle beam lithography comprising:
an input device capable of receiving a set of circular patterns to be formed on a reticle;
a computation device capable of determining a set of shots that can be used to form the set of circular patterns, wherein the dosages of the shots may vary with respect to each other; and
an output device capable of receiving the determined set of shots, including the dosages.
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This application: 1) is a continuation-in-part of U.S. patent application Ser. No. 12/202,364 filed Sep. 1, 2008, entitled “Method and System For Manufacturing a Reticle Using Character Projection Particle Beam Lithography”; 2) is a continuation-in-part of U.S. patent application Ser. No. 12/473,241 filed May 27, 2009, entitled “Method for Manufacturing a Surface and Integrated Circuit Using Variable Shaped Beam Lithography”; 3) claims priority from U.S. Provisional Patent Application Ser. No. 61/224,849 filed Jul. 10, 2009, entitled “Method and System for Manufacturing Circular Patterns On a Surface And Integrated Circuit”; and 4) is related to Fujimura, U.S. patent application Ser. No. ______, entitled “Method and System For Forming Circular Patterns on a Surface” (Attorney Docket No. D2SiP018b) filed on even date herewith; all of which are hereby incorporated by reference for all purposes.
The present disclosure is related to lithography, and more particularly to the design and manufacture of a surface which may be a reticle, a wafer, or any other surface, using charged particle beam lithography.
In the production or manufacturing of semiconductor devices, such as integrated circuits, optical lithography may be used to fabricate the semiconductor devices. Optical lithography is a printing process in which a lithographic mask or photomask manufactured from a reticle is used to transfer patterns to a substrate such as a semiconductor or silicon wafer to create the integrated circuit. Other substrates could include flat panel displays or even other reticles. Also, extreme ultraviolet (EUV) or X-ray lithography are considered types of optical lithography. The reticle or multiple reticles may contain a circuit pattern corresponding to an individual layer of the integrated circuit and this pattern can be imaged onto a certain area on the substrate that has been coated with a layer of radiation-sensitive material known as photoresist or resist. Once the patterned layer is transferred the layer may undergo various other processes such as etching, ion-implantation (doping), metallization, oxidation, and polishing. These processes are employed to finish an individual layer in the substrate. If several layers are required, then the whole process or variations thereof will be repeated for each new layer. Eventually, a combination of multiples of devices or integrated circuits will be present on the substrate. These integrated circuits may then be separated from one another by dicing or sawing and then may be mounted into individual packages. In the more general case, the patterns on the substrate may be used to define artifacts such as display pixels or magnetic recording heads.
In the production or manufacturing of semiconductor devices, such as integrated circuits, maskless direct write may also be used to fabricate the semiconductor devices. Maskless direct write is a printing process in which charged particle beam lithography is used to transfer patterns to a substrate such as a semiconductor or silicon wafer to create the integrated circuit. Other substrates could include flat panel displays, imprint masks for nano-imprinting, or even reticles. Desired patterns of a layer are written directly on the surface, which in this case is also the substrate. Once the patterned layer is transferred the layer may undergo various other processes such as etching, ion-implantation (doping), metallization, oxidation, and polishing. These processes are employed to finish an individual layer in the substrate. If several layers are required, then the whole process or variations thereof will be repeated for each new layer. Some of the layers may be written using optical lithography while others may be written using maskless direct write to fabricate the same substrate. Eventually, a combination of multiples of devices or integrated circuits will be present on the substrate. These integrated circuits are then separated from one another by dicing or sawing and then mounted into individual packages. In the more general case, the patterns on the surface may be used to define artifacts such as display pixels or magnetic recording heads.
In semiconductor manufacturing, reliably manufacturing contacts and vias is difficult and important, especially when optical lithography is used to manufacture patterns smaller than 80 nm half pitch, where half pitch is one-half of the minimum contact or via size plus one-half of the minimum required space between contacts or vias. Contacts and vias connect a conductive material on one layer to another conductive material on another layer. In older technology nodes which were relatively larger than currently-popular technology nodes, attempts were made to manufacture square vias and contacts on the wafer. Square contacts and vias are desirable so as to maximize the amount of area that connects between the conductive material in the below layer and the conductive material in the above layer. But with decreasing feature sizes, it has become prohibitively expensive or impractical to create large numbers of square patterns on the semiconductor wafer. Especially at 80 nm half pitch and below, semiconductor manufacturers target forming near-circles on the wafer, when viewed from above, which create nearly cylindrical contacts or vias. The design data that specifies the desired wafer shape still specifies the desired shape as a square. However, the manufacturers and designers alike work with the assumption that limitations of the optical lithographic process will cause the actual resulting shape to be a near-circle on the wafer. The generalized case of this effect for all shapes is sometimes referred to as corner rounding.
A significant advantage to the conventional practice of specifying contacts and vias as squares in the design data is that square patterns can be formed relatively quickly on a reticle. The use of square patterns for contacts and vias on the reticle and photomask, however, make the manufacturing of vias and contacts on the semiconductor device more difficult. It would be advantageous to eliminate the manufacturing difficulties associated with using square patterns on a photomask for contacts and vias, particularly for half-pitches less than 80 nm.
A method for manufacturing a semiconductor device using a photomask and optical lithography is disclosed, wherein circular patterns on the semiconductor wafer are formed by using circular patterns on the photomask, which is manufactured using a charged particle beam writer. In one embodiment, circular patterns of varying sizes have been formed on the photomask using a single character projection (CP) character, by varying the charged particle beam dosage.
A method for fracturing circular patterns is also disclosed, either using circular CP characters or using variable shaped beam (VSB) shots wherein the union of the plurality of VSB shots is different than the set of desired patterns.
These and other advantages of the present disclosure will become apparent after considering the following detailed specification in conjunction with the accompanying drawings.
There is an important concept called Mask Error Enhancement Factor (MEEF) in semiconductor lithography. In a typical semiconductor manufacturing process using photomasks, the photomasks are four times the dimensions of the wafer. For example, a 50 nm target shape on a surface appears as a 200 nm shape on the photomask. If MEEF was 1.0, a 4 nm offset error on the photomask would translate to a 1 nm offset on the wafer. However, a typical MEEF for lines and spaces, such as on interconnect or wiring layers, is 2. For contact layers, a typical MEEF is 4, which means that a 4 nm offset error on the photomask translates to a 4 nm offset on the wafer. In advanced technology nodes with contacts layers which are less than 80 nm half-pitch, a MEEF as high as 10 may be projected. In such a case a 4 nm offset on the photomask translates into a 10 nm offset on the wafer. Thus photomasks, and particularly photomasks for contact layers, are required to be extremely accurate in order that the MEEF-multiplied error on the surface does not exceed the maximum-allowed error.
One known method for improving MEEF is the so-called perimeter rule. The perimeter rule states that for a given enclosed shape, a higher ratio of the shape's perimeter to the shape's area results in a larger MEEF. In semiconductor manufacturing, it is most important in the lithography step to expose the resist with the right amount of total energy for each shape on the mask. Therefore, for each pattern or shape, accuracy is more important for the total area than the other dimensions of the pattern or shape. Various sources of error in the semiconductor manufacturing processes act on the perimeter, which are the set of edges that enclose the shape. These edges may move inward or outward compared to the desired location. When the ratio of the perimeter to the area is relatively large, the entire perimeter moving inwards by a given distance, say 1 nm, shrinks the enclosed area by a larger amount than if the ratio was relatively smaller. Because total area is total energy, and the total energy is critical for each shape, a smaller ratio is desired for every shape. Among geometric shapes, a circle has the smallest perimeter per unit area of any shape. Therefore, circular shapes or patterns will have a smaller MEEF than any non-circular shape. Nearly-circular shapes will have a MEEF that is almost optimal.
Mask making today is done by either a laser-based mask writer, or a charged particle beam mask writer, such as an electron beam mask writer. Today's production tools for the most advanced technology nodes with the smallest features below 80 nm half pitch are all done using an electron beam mask writer using variable shaped beam (VSB) technology with a high voltage (50 KeV and above) electron gun. Conventional reticle or mask writing includes a step of fracturing all desired mask shapes into constituent rectangles and 45 degree triangles of a certain size limit (for example, between 1 nm wide and 1000 nm wide) such that the union of all shapes is the original shape, perhaps within a certain minimum threshold, and such that the constituent shapes do not overlap. The fractured shapes are individually written by the electron beam mask writer as VSB shots. Reticle writing typically involves multiple passes whereby the given shape on the reticle is written and overwritten. Typically, two to four passes are used to write a reticle to average out errors, allowing the creation of more accurate photomasks. Conventionally, within a single pass the constituent shapes do not overlap. In reality, because electron beam mask writers are not perfectly accurate, some VSB shots that were designed to abut will overlap slightly. Also, between some VSB shots that were designed to abut there will be minute gaps. The electron beam mask writer's placement accuracy and the semiconductor design are carefully coordinated to avoid problems that arise due to these overlaps and gaps. The problems that arise are minimal particularly for small errors of 1 nm or below because the electron beam being transmitted has a natural blurring radius (roughly of 20-30 nm size), causing a Gaussian distribution of transferred energy beyond the drawn edges of the shapes. The dose for each of the VSB shots is assigned in a later and separate step. The dose determines the shutter speed, or the amount of time that the electrons are being transmitted to the surface. Proximity Effect Correction and other corrective measures determine what doses should be applied to each VSB shot to make the resulting photomask shape as close to the originally-desired photomask shape as possible.
Conventionally, one VSB shot is required for forming a square contact or via pattern. Forming a circular pattern on a reticle using conventional mask writing technology requires many VSB shots. Increasing the number of VSB shots has a direct impact on the amount of time required to write the reticle, which directly translates to photomask cost. Since for a typical integrated circuit design, many million contact and via patterns must be formed, the formation of circular contact or via patterns on a reticle using conventional VSB shots is not considered practical.
Two-dimensional maps of dosages known to be generated on a surface by single charged particle beam shots or combinations of charged particle beam shots are called glyphs. Each glyph may have associated with it the position and shot dosage information for each of the charged particle beam shots comprising the glyph. A library of glyphs may be pre-computed and made available to fracturing or mask data preparation functions. Glyphs may also be parameterized.
It should be noted that, as is common in integrated circuit design, a two-dimensional shape, such as a circle, refers to a shape on the semiconductor wafer as viewed from top-down. In the case of contacts and vias, the actual three-dimensional manufactured shapes may be cylindrical or nearly cylindrical.
The methods set forth herein for forming circles on surfaces such as reticles using either VSB shots or circular CP characters may also be used to form patterns directly on substrates such as silicon wafers, using maskless direct write. It should be noted that MEEF is not an issue for direct writing.
The techniques of the present disclosure may also be used when the desired pattern to be formed on a surface is nearly-circular.
The formation of circles on a surface can be approximated by a non-circular shape such as a polygon. Where a circle is desired on a surface or on a substrate such as a silicon wafer, the result may be a near-circle, such as a curvilinear shape which closely resembles a circle.
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The various flows described in this disclosure may be implemented using general-purpose computers with appropriate computer software as computation devices. Due to the large amount of calculations required, multiple computers or processor cores may also be used in parallel. In one embodiment, the computations may be subdivided into a plurality of 2-dimensional geometric regions for one or more computation-intensive steps in the flow, to support parallel processing. In another embodiment, a special-purpose hardware device, either used singly or in multiples, may be used to perform the computations of one or more steps with greater speed than using general-purpose computers or processor cores. The optimization and simulation processes described in this disclosure may include an iterative optimization process such as with simulated annealing, or may constitute solely a constructive, greedy, deterministic or other process without iterative improvement.
All references in this disclosure to circles should be interpreted to also include near-circles. Similarly, all references to circular patterns, circular apertures, circular characters, or circular CP characters should be interpreted to also include nearly-circular patterns, apertures, characters, or CP characters. Also, all references to cylinder should be interpreted to include near-cylinder, and all references to cylindrical should include nearly-cylindrical.
While the specification has been described in detail with respect to specific embodiments, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. These and other modifications and variations to the present system and method for manufacturing circular patterns on a surface or method for manufacturing an integrated circuit or method and system for fracturing or mask data prepraration may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present subject matter, which is more particularly set forth in the appended claims. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to be limiting. Thus, it is intended that the present subject matter covers such modifications and variations as come within the scope of the appended claims and their equivalents.